This invention relates to an ethylene polymer foam structure having enhanced processability and physical properties. The invention further relates to a process for making the foam structure.
Isobutane has become a preferred blowing agent for making ethylene polymer foam structures because of its zero ozone depletion potential and relatively high degree of processability and foamability, which can result in end products having generally desirable physical properties. A concern with using isobutane is that it is a volatile organic compound, which may be subject to environmental regulation. Another concern is that under some conditions, sometimes incomplete or inconsistent compressive recovery can be observed in end product foam structures at certain critical points in time in the product life cycle.
A means of reducing the volatile organic content of a blowing agent containing isobutane is to replace part or all of the isobutane with a hydrofluorocarbon, which may not cause the same measure of environmental concern. Suitable hydrofluorocarbons include 1,1,1-trifluoroethane (HFC-143a) and 1,1,1,2-tetrafluoroethane (HFC-134a).
Replacing isobutane with hydrofluorocarbons can present processing and extrusion problems because of the relatively low solubility of hydrofluorocarbons in melts of ethylene polymers. The processing and extrusion problems would be expected to take the form of a relatively narrow range or "window" of foaming temperatures or less than desirable physical properties in the end product. Undesirable physical properties can include poor skin quality, small cell size, high foam density, and small cross-section.
Foam structures blown with a blowing agent comprised entirely of isobutane sometimes do not fully recover after compression, to which the foam structures are often subjected to during die cutting. After compression and release from compression, foam structures blown entirely with isobutane recover a substantial proportion (i.e. about 88-95 percent by volume or thickness) of their initial volume prior to compression, but may then shrink to some degree (i.e. about 3-10 percent by volume or thickness) for an extended period of time before expanding and re-gaining a substantial proportion of the initial volume. This temporary shrinkage is a problem for cushion packaging end users or customers because the shrinkage typically occurs while the foam structure is being used as cushion packaging. The shrinkage results in an undesirable loose fit between the packaging material and the article or articles being packaged.
It would be desirable to have an ethylene polymer foam structure and process for making wherein the use of isobutane as a blowing agent is reduced. It would further be desirable to make a foam structure under a relatively wide range of processing conditions with desirable foam physical properties. It would further be desirable to make such a foam structure with improved compressive recovery.